Good, reliable communications among personnel engaged in hazardous environmental activities, such as fire fighting, are essential for accomplishing their missions while maintaining their own health and safety. Working conditions may require the use of a pressurized air delivery system such as, for instance, a Self Contained Breathing Apparatus (SCBA) mask and air delivery system. However, even while personnel are using such pressurized air delivery systems, it is desirable that good, reliable communications be maintained and personnel health and safety be effectively monitored.
FIG. 1 illustrates a simple block diagram of a prior art system 100 that includes a pressurized air delivery system 110 coupled to a communication system 130. The pressurized air delivery system typically includes: a breathing mask 112, such as a SCBA mask; a mask air regulator 118; a high pressure hose 120 connecting the regulator 118 to a low-air detection alarm device 122; and a high pressure air cylinder/tank 126 which supplies air to the system through an air cylinder supply valve 124. The low-air alarm device 122, usually mechanical in nature, produces an acoustic periodic alarm signal indicating when the supply of air in the tank is low. This device is usually attached to the air tank near the air tank supply valve 124. This low-air alarm signal is referred to herein as the Low-Air Alarm (LAA) noise.
Depending upon the type of air delivery system 110 being used, the system 110 may provide protection to a user by, for example: providing the user with clean breathing air; keeping harmful toxins from reaching the user's lungs; protecting the user's lungs from being burned by superheated air inside of a burning structure; and providing protection to the user from facial and respiratory burns. Moreover, in general the mask is considered a pressure demand breathing system because air is typically only supplied when the mask wearer inhales.
Communication system 130 typically includes a conventional microphone 132 that is designed to record the speech of the mask wearer and that may be mounted inside the mask, outside and attached to the mask, or held in the hand over a voicemitter port (a thin metal plate designed to pass speech sounds from inside the mask to the outside with minimal attenuation) on the mask 112. Communication system 130 further includes a communication unit 134 such as a two-way radio that the mask wearer can use to communicate his speech, for example, to other communication units. The mask microphone device 132 may be connected directly to the radio 134 or through an intermediary electronic processing device 138. This connection may be through a conventional wire cable (e.g., 136), or could be done wirelessly using a conventional RF, infrared, or ultrasonic short-range transmitter/receiver system. The intermediary electronic processing device 138 may be implemented, for instance, as a digital signal processor and may contain interface electronics, audio amplifiers, and battery power for the device and for the mask microphone.
There are some shortcomings associated with the use of systems such as system 100. These limitations will be described, for ease of illustration, by reference to the block diagram of FIG. 2, which illustrates the mask-to-radio audio path of system 100 illustrated in FIG. 1. Speech input 210 (e.g., Si(f)) from the lips enters the mask (e.g. a SCBA mask), which has an acoustic transfer function 220 (e.g., MSK(f)) that is characterized by acoustic resonances and nulls. These resonances and nulls are due to the mask cavity volume and reflections of the sound from internal mask surfaces. These effects characterized by the transfer function MSK(f) distort the input speech waveform Si(f) and alter its spectral content. Other sound sources are noises generated from the breathing equipment including regulator inhalation noise and low-air alarm noise 230 that also enters the mask and is affected by MSK(f). Another transfer function 240 (e.g., NPk(f)) accounts for the fact that the noise is generated from a slightly different location in the mask than that of the speech. The low-air alarm noise 230 may be conducted from the alarm device into the mask though the air but primarily through the air supply hose. The speech and noise S(ƒ) are converted from acoustical energy to an electronic signal by a microphone and amplifier, 250, which has transfer function (e.g., MIC(f)), producing an output signal 260 (e.g., So(f)) that may then be input into another device for further processing or directly into a radio for transmission.
Returning to the shortcomings of systems such as system 100, an example of such a shortcoming relates to the generation by these systems of loud acoustic noises as part of their operation. More specifically, these noises can significantly degrade the quality of communications, especially when used with electronic systems such as radios. One such noise that is a prominent audio artifact introduced by a pressurized air delivery system, like a SCBA system, is the low-air alarm noise, which is illustrated in FIG. 2 as box 230.
The low-air alarm (LAA) noise occurs as a low frequency, periodic, pulsatile harmonic-rich broadband noise generated by an alarm device coupled to the pressurized air delivery system (FIG. 1, 122). The alarm noise is designed to be generated when the air tank pressure drops below a specified level, indicating that the air supply is low (generally when about five minutes of breathable air remains in the tank). This noise is picked up by the mask communications system microphone along with ensuing speech, and has about the same energy as the speech. The LAA noise, once started, is continuous until the air in the tank runs out, and a SCBA wearer has little or no control over the alarm noise. The broad spectrum of the noise masks any concurrent speech signal and interferes with communications. The LAA noise can severely affect communication systems that use digital radios. Certain widely used digital codecs, especially ones based on a parametric speech model, are very sensitive to periodicities in a signal and their operation can be severely corrupted by certain periodic noises. In addition, the LAA noise is, in general, very annoying to a listener.
Thus, there exists a need for methods and apparatus for effectively detecting and attenuating low-air alarm noise that corrupts audio communication in a system that includes a pressurized air delivery system coupled to a communication system.